In an HVAC cable, electric field distribution depends on capacitances, which are dominated by permittivity of the insulation material that is stable with temperature. A specified is therefore not required when testing such cables. By contrast, field distribution in an HVDC cable is determined by resistivity of the insulation material, which varies greatly with temperature. This results in the need to control temperature difference, during thermal stresses represented by the load cycle in a test. In operation, an HVDC cable also experiences SI/LI overvoltage, which is superimposed onto the DC service voltage. As such, to properly simulate an in-service condition, superimposing a switching surge and a lightning transient onto the operating DC voltage is a standard requirement.
This edited 2017 contribution to INMR by Hong He and W. Sloot of KEMA Labs in the Netherlands described a test circuit with two separate test systems – a DC generator and an impulse generator, connected with a test cable system located between the two. The test cable system consists of three pieces of 50 m XLPE HVDC cable assembled with two outdoor terminations and two joints. One major complexity from this arrangement is preventing damage to each test system due to presence of the other. Therefore, a sphere gap is installed to block DC voltage from the impulse generator with a damping impedance set to protect the DC generator from very fast transients.
Thermal Physical Model
A temperature difference arises across the cable and temperature distribution inside can either be measured by thermocouples or calculated by a numerical model. Fig. 1 shows an equivalent thermal circuit for the cable, where
Q, heat transfer rate (heat flow), W;
θc, θcs, θi, θis, θis, θamb, temperature of conductor, conductor screen, insulation, insulation screen, sheath and ambient, °C;
Δθcs, Δθi, Δθis, temperature drop across conductor screen, insulation and insulation screen respectively, K;
Δθc-is, temperature difference from conductor to insulation screen, K;
Tcs, Ti, Tis, thermal resistance of conductor screen, insulation, insulation screen respectively, K·m/W;
Ts, thermal resistance in total from metal sheath to over sheath, K·m/W;
Ttm, thermal resistance of external thermal insulation material, K·m/W;
Temperature difference across conductor screen, insulation and insulation screen is calculated as follows:
Eq (5) is the simple formula for calculation of Δθmax. Since the surroundings also play a role, the main temperature drop in a cable is between the cable surface (sheath) and environment (θs – θamb). In order to control Δθi within the limit of Δθmax, external thermal insulation material (e.g. plastic bubble) is applied to minimize influence of the environment. This thermal insulation material helps adjust Ttm based on different cable design and surrounding condition.
Load Cycle & Temperature Measurement
In order to monitor the thermal environment of HVDC cable system, thermocouples were placed at a total of more than 20 different locations on a reference cable and a test cable. The reference cable must be installed in close proximity with the test cable to ensure equal thermal environment. Conductor temperature of the reference cable (based on rated maximum θc, e.g. 70°C) was monitored to guide the same current applied to the test cable simultaneously. By applying Eq (5), specified Δθmax was calculated and monitored during the load cycle. Fig. 2 shows a typical 24 hours load cycle.
Figs. 3 and 4 schematically demonstrate the cable cross-section (e.g. 2500 mm2) and locations of measured temperature respectively.
The experimental setup included a DC generator, an impulse generator and the test object – a 320 kV DC cable system including HVDC XLPE cable (copper conductor and XLPE insulation, nominal capacitance of 0.26 µF/km), two outdoor oil terminations and two pre-moulded joints with shield interruption. Figs. 5 and 6 show a schematic of the test circuit and an actual test set-up. Each test system was equipped its own divider to measure DC voltage (Divider 1 in Fig. 5) and SI/LI (Divider 3 in Fig. 5) respectively. A combined divider (Divider 2 in Fig. 5) was applied to measure the superimposed withstand voltage subjected directly to the test object.
A pair of spheres (diameter of 0.5 m, SG2 in Fig. 5) was installed in vertical position, the lower sphere directly fixed to the outdoor terminations. The DC generator was also connected to the lower sphere with the protection water resistor (10 MΩ, R1 in Fig. 5) and the blocking impedance (35 mH, L1 in Fig. 5) in between. The upper sphere was separated from the lower sphere and the cable by an air gap and connected to the impulse generator. A surge arrester (A1 in Fig. 5) was installed for extra protection of the DC generator.
Accurate measurement of voltage waveforms and self-ignition of sphere gap are the two essential aspects for a superimposed impulse test. By means of a sphere gap, this gap must ignite at a certain level and transfer the SI/LI to the cable system. The gap distance has to be considered and adjusted. Under the same polarity of two voltages (e.g. + UDC and + USI, a unipolar stress), the breakdown voltage of the sphere gap (Ubd unipolar = UDC – USI ≥ UDC , thus USI ≥ 2 UDC), in contrary to a bipolar stress (e.g. + UDC and – USI), the voltage (Ubd bipolar = UDC – (– USI) ≥ UDC , thus absolute USI ≥ 0). Therefore under the bipolar stress, there is not critical for breakdown the air gap; however under the unipolar stress, it is a complex procedure due to the Ubd unipolar. Furthermore, these calibration steps are necessary to avoid overshooting the test object . The distance of sphere gap needs to be adjusted corresponding with the calibration of impulse voltages from low scales (50%, 65% and 80% of the test voltage) up to full scale (100% of the test voltage).
Analysis of Measured Results
The three measured voltages were obtained with three dividers D1, D2, D3 (see Fig. 5) during testing. The DC voltage measured by Divider 1 via a digital voltage meter and the full scale of DC voltage should be 320 kV. The SI/LI voltages were overserved by Divider 3 and the peak voltages should be 710 kV and 740 kV respectively (see Fig.7). DC voltage superimposes with the same polarity of SI as shown in Fig. 8. It was further found that DC voltage superimposes with opposite polarity SI and LI in Figs. 9 and 10 respectively.
In Fig. 7, Divider 3 measured a spike in front of the SI wave shape, which is the time delay of the protection sphere gap, it was until over the breakdown voltage, then sphere gap was conducting and the cable system was connected at that moment to the impulse generator.
As observed in Fig. 8, a systematic DC offset error occurs during the tests. Divider 2 was calibrated by DC voltage and Impulse voltages respectively. For combined voltages, a correction factor is needed (i.e. a factor of 0,78). This offset leads to a difference of peak voltages of impulses measured by Divider 3 and Divider 2.
It is noticed that, once the arc between the spheres was ignited, the voltage on the cable system followed the voltage at the terminal of the impulse generator. Thus the voltage on the cable system in extreme short time (hundreds or thousands of µs) discharged via impulse generator to extinguish voltage of the arc (visible in graphs), then it would take short recover time (few hundreds of ms) to recharge to the DC voltage (invisible in graphs due to limitation of data collection). In the meantime, the protection resistors sustained the full difference of impulse voltage and DC test voltage.
Calculation of Δθmax is obtained based on the heat transfer method. To maintain constant surface temperature of the cable, as needed to guarantee a specified Δθmax, practical methods include controlling load current through the conductor, maintaining constant ambient temperature or applying an external thermal insulation material. Test set-up using a sphere gap has been developed for application of SI/LI superimposed on a DC voltage. Correct gap adjustment is essential for unipolar stress condition. Otherwise, under certain circumstances high-frequency oscillations might occur. Further experimental work focuses on a method to control cable sheath temperature and developing three-channel software with extended data capture time (up to the recover time of DC voltage). A protection capacitor is an alternative to a sphere gap to realize a superimposed withstand test.
 Cigre 496: Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500kV, April 2012.
 IEC 62895 Edition 1.0: 2017-05: High voltage direct current(HVDC) power transmission – Cables with extruded insulation and their accessories for rated voltages up to 320 kV for land applications – Test methods and requirements.
 E. Pultrum, W. Sloot, J. Fernandez, R. P. P. Smeets, “High-voltage cable testing: type test experiences and new insights into pre-qualification”, CEPSI, 2016.
 IEC 60060-1 Edition3.0: 2010-09: High-voltage test techniques – Part 1: General definitions and test requirements.